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Nuclear Magnetic Resonance Spectroscopy

NMR SpectroscopyMethod for determining the structure of organic moleculesinterpretationsample preparationdata acquisition

3NMR Instrument

Electrons shield H from external magnetic field.This results in small difference in the position of absorptions.

To run a spectrum - place the sample and solvent in a NMR tube. Place the tube between the poles of a powerful magnetIn the original type of instrument Bo was varied. When E matches the energy of the radiation, light was absorbed. (The compound was said to be in resonance with the radiation.)On the older instrument, it took several minutes to get spectrum. Newer instruments hold Bo constant and supply a pulse of radiation that excites all nuclei simultaneously. This generates a complex signal which can be turned into a spectrum by Fourier transform. This process is much faster, allowing computer averaging to be used to get a better signal.The exact E for a H depends on the local environment. Electrons shield the H from the external magnetic field. This requires a slightly larger magnetic field to obtain the same E. As a result, different Hs appear at slightly different positions on the chemical shift scale. The difference is small, about 2500 Hz on a 200 MHz instrument (0.001%), so we need an instrument with very high resolution.

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Theory1H and 13C have atomic spin numbers of 1/2Have two energy levels in an applied magnetic fieldaligned against the field: higher energyaligned with the field: lower energyDifference in energy is quantized

Radio frequency region electromagnetic radiation absorbed Frequencies absorbed depend on the environment of the nuclei and the applied magnetic field strength

Interpretation of Proton NMR SpectraStructure from the spectrumspectra of compounds with unknown structures structure determination usually requires complementary spectral dataSpectrum from the structureused to verify productscompare to published spectracomputer model of hypothetical spectrum

Signals Due to Equivalent NucleiNuclei--chemically and magnetically equivalentreplacing any equivalent atom of a grouping with another atom gives the same compoundatoms are often equivalent by fast rotationEthyl Iodide has two proton absorption signals from two sets of equivalent protons

60 MHz Proton NMR Spectrum of Ethyl Iodide

250 MHz Proton NMR Spectrum of Ethyl Iodide

The Chemical ShiftResults from the effective magnetic field around nucleidepends on structurehybridization electronegativity of neighboring atoms

Each absorption signal in the NMR spectrum can be assigned to a particular structural featureInconsistencies sometimes occur Tables of chemical shift values are generally representative

Question: if u have a proton signal at 125 Hz in 60 MHz spectrometer, what's the position of this signal if we use 300MHz apparatus?

= 125/60 = 2.1 ppm Chemical Shift Data

Inductive effect influence chemical shiftTypes of Hydrogens on Carbon

Types of Hydrogens on Carbon

Splitting Patterns and Coupling ConstantsHelpful in determining types of hydrogen on carbons adjacent to the carbon with the protons in resonance Chemically equivalent protons on neighboring carbons typically give a pattern of lines containing one more line than their total This splitting pattern of lines represents the absorption signal for proton(s) in resonance Peak intensities follow the binomial expansion

Pascals Triangle

Coupling Constants (cont.)The peaks at 1.83 ppm and 3.20 ppm have been expanded and plotted on the left side of the spectrum (line spacing in the expanded region is equivalent because the protons are coupled)

Coupling ConstantsQuartet-triplet combination strong indicator for ethyl groupThe spacing between lines in an absorption signal caused by protons on a neighboring carbon is called the coupling constant (constants are measured in Hertz)The spacing between the lines in Hertz will be the same for coupled groups on adjacent carbons

abc0.96No. of neighboring protons2Splitting (n+1)t(3)

abcNo. of neighboring protonsSplitting (n+1)

No. of neighboring protonsSplitting (n+1)

No. of neighboring protonsSplitting (n+1)

No. of neighboring protonsSplitting (n+1)28Propose a structure for the compound of molecular formula C4H10O whose proton NMR spectrum follows.

Solved Problem 3Solution29

Solved Problem (Continued)Solution (Continued)Integrals of Absorption SignalsHeight of stair-step line is proportional to the area under the signal lineThe area of each absorption signal is directly proportional to the number of hydrogen atoms causing that signal

Interpreting NMR SpectraUse a combination of approaches to interpret a spectrumComplimentary spectral data can provide a molecular formula and identify some functional groups on a structureNMR spectra permit a back-and-forth analysis to correlate molecular structure with absorption signalsA structure is verified when all of the absorption signals and patterns are accounted for

DeuteriumD = 2H - an isotope of 1Hinvisible in 1H NMRused in solventsCDCl3 (deuteriochloroform)

(perdeuterioacetone)Why we use TMS as a reference?

1. It contains 12 equivalent protons so it gives strong signal 2. Its protons are highly shielded in comparison with organic cpd, so its signal occurs at higher field than other protons 3. inert 4. volatile 5. soluble in most organic solvents

Filtration of NMR Samples Through a Pasteur PipetteCan also use two pipettes and gravity filtration

Two-Dimensional COSY NMR Spectrum

GOOD LUCK